<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            font-size: 10px;
        }
        .a {
            /* 32px */
            font-size: 2rem;
        }

        .a span {
            font-size: 5rem;
        }
        .a b {
            font-size: 3rem;
        }
    </style>
</head>
<body>

    <!-- 
        rem :  root em   永远以根元素的字体大小作为参照    html

     -->
    
     <div class="a">
        哈哈
        <span>呵呵 <b>哈哈</b></span>
     </div>
    
</body>
</html>